This invention relates generally to methods of torque transmission and, more particularly, to an electrorheological fluid coupling for controlling the magnitude of torque delivered from an input torque supply to an output drive device.
The principle of torque transmission control fundamentally involves the concept of controllably changing the torque-speed ratio or mechanical advantage between a rotating input shaft and a rotatable output shall. In an automotive setting, this concept is most notably used in an automotive transmission, a device which transmits engine rotation and power to the drive wheels at various ratios. The main function of the transmission is to enable the vehicle to accelerate from a standstill to maximum speed through selected gear ratios while the engine continuously operates within its most effective and efficient range.
Traditionally, the magnitude of torque delivered from a torque supply source to a final drive device has been controlled through the use of mechanical or frictional braking or by simply controlling the rotational speed of the motor or engine. More recently, viscoelastic fluids have also been utilized to controllably transfer torque. In these fluid-based systems, a pair of wetted surface plates, one connected to the torque supply source by a torque input shaft and the other connected to the drive device by an output shaft, are immersed in a viscoelastic fluid medium. The viscous properties of the fluid cause a "fluidic couple" to form as a pseudo-solid between the two wetted surface plates. This couple facilitates torque transmission between the plates by forcing the plate attached to the output shaft to rotate in response to a rotation of the plate adjoining the rotating input shaft. This ultimately results in the transfer of torque from the supply to the final drive device and is the basic operating principle behind the viscous coupled transmission or transfer case.
While viscoelastic fluids have provided some distinct advantages over more conventional mechanical and rotational mechanisms for torque transmission control, there are several inherent difficulties in this type of system. With normal viscoelastic fluids, torque transmission between the wetted plates is governed solely by the viscoelastic properties or the medium. Normal viscoelastic fluids have a very fixed relationship between their rheological properties and various environmental parameters. At any given temperature, the viscosity, coherence and maximum sheer coefficients of the fluid will be constant. However, as temperature increases, usually due to friction or interstitial molecular dynamics (shear thinning within the fluid), the torque transmission efficiency of the viscoelastic fluid normally declines due to reduced coherence and/or viscosity. The net result is usually increasing "fluid slip", eventually to the point where the pseudo-solid cracks or fractures, resulting in torque transmission pulsing or cessation.
The present invention attempts to provide an improved torque transmission control system by defining a fourth mechanism of torque magnitude control utilizing an electrorheological fluid coupling. Electrorheological fluids are suspensions which exhibit a change in their flow characteristics and viscosity upon the application of an electric charge or electric field. With these fluids, the relationship between rheological and environmental parameters can be more precisely controlled over a specific bandwidth of operating conditions than with normal viscoelastic fluids.
Through the generation of a variable electric charge or field by the application of an electrical current or voltage to the fluid, the rheological properties of the electrorheological fluid are precisely controllable. This enables constant and semi-infinite torque magnitude control over wide ranges of rotational speeds, input torque magnitudes and operating parameters. By controllably altering the amount of electrical energy applied to a contained electrorheological fluid medium in which a pair of wetted surface plates are immersed, the system of the present invention controls the magnitude of torque transmitted between these plates and, ultimately, from a torque supply to a final drive device, in a manner not heretofore possible with ordinary viscoelastic fluids.
Additional advantages and features of the present invention will become apparent to one skilled in the art from the following description and claims, taken in conjunction with the accompanying drawings.